586 EXPLORATION GEOPHYSICS 



current is plotted against the distance from an electrode {x) as a fraction 

 of the total separation (L) . 



If the area under investigation is not homogeneous, the magnetic 

 field produced at the surface of the earth is not given by the simple for- 

 mula derived above. When direct current or very low frequency ener- 

 gizing current is employed, the magnetic effects observed at the surface 

 of the earth depend on the current paths, and these in turn depend on the 

 relative conductivities of the materials constituting the subsurface. For 

 example, if the subsurface consists of several layers having different 

 conductivities, the effective depth of current penetration and the current 

 paths will depend principally on the values of the conductivities and on the 

 separation of the energizing electrodes. Thus, precisely as in the resistivity 

 methods, variation of the electrode separation permits depth control. 



The resolving power of the method however is greater than that of 

 the resistivity methods. This increased resolving power is due largely 

 to the following factors: 



(1) The near-surface effects are minimized since a considerable por- 

 tion of the electromagnetic field being measured is due to the deeper current 

 flow. In the resistivity methods, the only potentials that are effective or 

 measured are those at the surface of the earth. 



(2) Near-surface effects diminish, due to the increased flow of cur- 

 rent to greater depths as the electrode spacing is increased. 



(3) Because the electromagnetic field strength decreases inversely 

 as the distance and the sine of the angle, the current flowing beneath the 

 point of measurement has a much greater effect than current flowing to 

 each side and farther removed from the instrument. 



Theoretically, it is possible to calculate the magnetic effects produced 

 by layered media in a manner similar to that used in the preceding section 

 by taking into account the curvature of the current paths. Practically, 

 however, recourse is usually had to an empirical interpretative technique 

 wherein observed electromagnetic anomalies are considered as diagnostic 

 with respect to subsurface inhomogeneities. That is, interpretation is 

 nearly always based on anomalous conditions as indicated by deviations 

 of the field strength from the theoretically normal values. In this respect 

 the electromagnetic data are interpreted by the same technique employed 

 in interpretation of resistivity data. Curves are usually plotted showing 

 the observed field strength per unit current versus : ( 1 ) electrode spacing, 

 (2) traverse distance at a constant energizing electrode spacing, or (3) 

 magnetometer position, with reference to the distance to the two fixed 

 electrodes. 



The third technique results in the most advantageous field procedure 

 where the relative depths to a marker bed or contact are to be mapped. 

 The electrode spacing is chosen to give the maximum effective current flow 

 within the general depth range being mapped. A curve is then plotted for 

 this electrode spacing, as shown in Figure 365. When the effective current 



